1. Field of the Invention
The present invention relates to a process of forming a semiconductor optical device type of semiconductor laser diode (LD) and an LD formed thereby.
2. Background Arts
Various prior documents have reported processes of forming an LD of the type of, what is called, a buried hetero-structure (BH). A typical process for a BH-LD includes steps of: (a) preparing a semiconductor stack that includes a lower cladding layer, an active layer, and an upper cladding layer; (b) forming a mesa by etching portions of the lower cladding layer, the active layer, and the upper cladding layer with a striped mask extending along a direction; (c) selectively growing a first burying layer so as to bury the mesa; (d) exposing a top of the mesa by partially melting the first burying layer; (e) growing a second burying layer on the first burying layer and the exposed top of the mesa; (f) exposing the top of the mesa by partially melting the second burying layer; and (g) growing the third cladding layer on the exposed top of the mesa, which exposes the upper cladding layer, and on the second burying layer. Opposing the conduction type of the lower cladding layer to the conduction type of the upper cladding layer, and matching the conduction type of the lower cladding layer with that of the second burying layer, and also matching that of the first burying layer with that of the upper cladding layer and the third cladding layer, a current confinement structure of the BH-LD may be formed.
As accelerating transmission speed in the optical communication system, an LD implemented therein has been strongly requested to be operable in high speed and to show excellent emission efficiency. In order to enhance the emission efficiency, an LD is necessary to reduce parasitic resistance and increase carrier confinement into the active layer. The carrier confinement in a BH-LD may be realized by narrowing a window formed by the burying layers above the mesa, which means that the upper cladding layer and/or the third cladding layer above the upper cladding layer inevitably increases parasitic resistance thereof. Enhancement of doping density by p-type impurities into the upper cladding layer and/or the third cladding layer above the upper cladding layer may reduce the parasitic resistance but zinc (Zn) atoms, which are typical p-type dopant for the cladding layers, show faster diffusion velocity. Zincs diffused into the active layer easily form non-radiative recombination centers, which degrades the emission efficiency of a BH-LD. Accordingly, an arrangement for reducing the parasitic resistance of a BH-LD except for increasing doping density of Zn has been demanded in the field.